The vertebral column is a flexible spinal column, extending along the back of vertebrate animals, which includes a series of bones termed the vertebrae. The major function of the vertebral column is protection of the spinal cord. It also provides stiffening for the body and attachment of the pectoral and pelvic girdles and many muscles. In humans an additional function is to transmit body weight in walking and standing. Each vertebra, in higher vertebrates, consists of a ventral body, or centrum, surmounted by a Y-shaped neural arch. This arch extends a spinous process or projection downward and backward that may be felt as a series of bumps down the back, and two transverse processes, one to either side, which provide attachment of muscles and ligaments. The centrums are separated by cartilaginous intervertebral disks, which help cushion shock in locomotion.
In general, these disks have two functions: to allow movement between pairs of vertebrae and to act as buffers against the shocks of running, jumping, and other activities that apply stresses to the spine. The disks also cooperate with paired synovial joints, one on each side of the vertebral arch, to facilitate complex multidimensional relative movement between adjacent vertebrae.
Surgical intervention is often required following degeneration of intervertebral disks. In many instances, spinal fusion is the desired result. The usual rationale for fusion is prevention of progressive apposition of the vertebral bodies with consequent subluxation of the facet joints, narrowing of the nerve route foramina, and development of arthrosis. For the most persistent cases of disabling back pain, direct bony fusions are often performed to stop the painful motion between vertebrae by permanently locking them to one another. However, in many cases it might be advisable to allow continued movement between adjacent vertebrae, which helps to prevent mechanical breakdown at nearby bone segments.
When dealing with ruptured disks, one surgical remedy is to partially remove the disk. Such removal satisfies only one particular part of the problem by removing the displaced disk material that has created pressure on a spinal nerve, but it does not restore the disk to its normal configuration. The result is a flattened and degenerative disk, which is usually accompanied by arthritis of the facet joints and might result in persistent low back pain. This might also lead to recurrent pressure on a nerve because the disk thinning associated with degeneration might lead to formation of a spur that results in renewed pressure on the nerve. Current fusions operations or bony decompressions are utilized, but they are only relatively satisfactory procedures because of the magnitude of any fusion operation and because spurs tend to recur when removed without an arthrodesis. Accordingly, there is a tendency for stenosis of the spinal canal or neural foramen to occur.
The present invention was developed in an effort to provide a surgeon with an element of control over the end result of such operations following disk removal. The desired result can be either arthrodesis (fusion) or arthroplasty (joint formation). Furthermore, recognizing that all surgical procedures will fail to achieve their intended results in a statistical percentage of situations, the present invention assures that an acceptable result will be achieved as an alternative to that desired. Briefly, where arthrodesis is the intended result, arthoplasty will be the alternative result, and where arthroplasty is the intended result, arthrodesis will be the alternative result. Thus, even though a planned fusion does not occur, the patient will be provided with a useful prosthetic joint between the affected vertebrae and, conversely, where joint formation was intended and not achieved, an acceptable solid fusion will be achieved in its place.
U.S. Pat. No. 4,501,269, issued on February 26, 1985, discloses an apparatus and process for immediate stabilization and subsequent promotion of fusion in bone joints. The disclosed implant is a metal cylinder or basket having fenestrations arranged about its cylindrical surface. Bone fragments are utilized within the basket to promote bone growth through the basket. While a cylindrical implant has limited usefulness in spinal operations where fusion is desired, the placement of a cylindrical basket is difficult because of the amount of surrounding bone that must be removed in order to properly insert the cylindrical object between adjacent vertebrae.
Recognition of the limitations relating to fusion procedures has led to identification of a need for an immediate artificial disk or fusion operation that can be done in the process of removing the disk to prevent further disk space narrowing and degeneration. It is this narrowing or loss of a shock absorber between adjacent vertebrae that leads to facet joint hypertrophy, spur development, etc. Furthermore, the required surgery should be of lower magnitude than present fusion surgery and should result in more predictably acceptable results. For example, present fusion operations require an additional one to two hours for carrying out the surgery after removal of the disk, and also require additional incisions to collect the required bone, unless a bone bank is utilized. Six to 12 months' time is needed for the fusion to become solid and this operation carries a 10 to 20 percent failure rate wherein pseudoarthrosis (evidenced by an unplanned, false joint) is the end result.
The present development is designed to meet the needs of a surgeon intending to produce either a multidirectional joint or a bony fusion, and provides the benefit of an acceptable alternative surgical result where the intended surgery fails. It assures immediate spacing of the vertebrae to remove verve pressure and an implant that can promote either arthrodesis or arthroplasty as indicated by the situation of a specific patient. This is accomplished by spreading of the vertebrae and implantation of a hollow spherical member having an interior cavity that can be filled with bone fragments and having an exterior wall with multiple fenestrations leading to the cavity. By selective choice of surgical procedures, the surgeon can promote fusion through the implant or tissue growth about it to form a defined joint. While one of these results may be the intended goal of surgery; the other, if it occurs, can be an acceptable alternative.